In recent years, low-power-consumption liquid crystal display (LCD) devices, which are thin and light in terms of weight and may also be driven with low value voltages, are widely used. In addition, the demand for increasing the screen size is higher year after year, and motion pictures such as TV images are required to be displayed on these devices. For that, interconnections need to be composed of materials that have low resistivity and high conductivity. In recent years, in response to the above requirements interconnections are expected to be made of new materials such as copper (Cu). Copper (Cu) has lower resistivity, namely higher conductivity compared to aluminum (Al) alloys.
According to the actual demand for large screens, materials for gate interconnections have changed from a molybdenum (Mo) alloy to an aluminum (Al) alloy or an aluminum clad, etc. Needless to mention that Aluminum (Al) has problems of hillocks, migrations, etc.
For example, as shown in Japanese Unexamined Patent Application Publication No. 2000-199054, interconnection materials composed of an aluminum-neodymium (Al—Nd) alloy is proposed, or anodically oxidized Al, Al claded by molybdenum (Mo) alloy, or double layered aluminum (Al) is used. In the case of aluminum-neodymium (Al—Nd) alloy the resistivity of interconnections is about 5.1 μΩ·cm, while the resistivity of pure aluminum (Al) is 2.5 μΩ·cm.
Therefore, interconnections composed of three layers of titanium/aluminum/titanium (Ti/Al/Ti) or molybdenum/aluminum/molybdenum (Mo/Al/Mo) are used as a countermeasure against the above mentioned problems of hillocks, migrations, etc. when pure aluminum (Al) is practically used as a material for interconnections. However, this multilayer structure brings about new problems such as an increase in the layer formation process.
On the other hand, nowadays, copper (Cu) is considered to be an attractive material for thin film transistor (TFT) electrodes or interconnections because it represents a low electrical resistance compared to the other materials used in TFT electrodes or interconnections. However, copper (Cu) has poor characteristics with regards to the adhesiveness with insulating layers, in particular, with glass, which is a material used for the substrate of TFT. In addition, copper (Cu) gets easily oxidized when formed on an insulating layer.
Accordingly, to resolve the above-mentioned problems, a technique employing alloyed copper interconnections is attempted in TFT-LCD devices. This technique aims at securing the adhesiveness to the glass substrate by reaction of alloy elements with the substrate forming a layer at their interface. In addition, this technique also aims at forming an oxide layer on a surface of copper (Cu), in which the alloy elements function as an oxidation-resistance layer with a low resistivity for the copper (Cu).
However, according to the proposed technique, characteristics that are aimed at are not sufficiently achieved. Electric resistance of copper (Cu) increases due to the fact that alloy elements are remaining in Cu layer, and therefore it could not show its advantage over conventional interconnection materials such as aluminum (Al) or aluminum alloy.
Further, as shown in Japanese Unexamined Patent Application Publication No. 2004-163901, in order to utilize a copper interconnection in TFT-LCD devices, another technique is proposed in that a molybdenum (Mo) alloy layer is interposed between the copper (Cu) layer and the substrate, thereby securing the adhesiveness and the barrier properties with the substrate.
However, according to this technique, the manufacturing process has an additional step for depositing molybdenum (Mo) alloy. In addition, the effective resistance of interconnections increases in this structure. Further, although a single layer of copper (Cu) is utilized for the source and drain electrodes of TFT-LCD devices, their stability remains under question.
Further, in Japanese Unexamined Patent Application Publication No. 2004-139057, in order to resolve the above mentioned problems with regards to the copper (Cu) interconnections, another technique is proposed in that a high-melting-point nitride such as tantalum nitride (TaN), titanium nitride (TiN) or tungsten nitride (WN) is formed around the copper (Cu). However, this technique arises other problems such as, for example, a new material for forming the barrier layer and even an additional process are required compared to the case where conventional materials are used for the interconnections. In addition, the effective resistance of the interconnection increases because a high-resistivity barrier layer is deposited thickly around the copper (Cu).
Further, Japanese Unexamined Patent Application Publication No. 2005-166757 discloses that an addition of one or more elements of magnesium (Mg), titanium (Ti) and chromium (Cr) to the copper (Cu) of the interconnections in TFT-LCD devices improves the adhesiveness as well as the oxidation resistance. However, another problem arises in that the interconnection resistance increases as the additional elements are remaining in the interconnections. In addition, the interconnection resistance increases because the additional elements reduce oxides in the substrate layer and the reduced elements diffuse into the interconnection.
Japanese Unexamined Patent Application Publication No. 2002-69550 discloses another technique, which tries to improve the oxidation resistance by adding silver (Ag) of 0.3 to 10 weight percent to the copper (Cu). However, in this case, the adhesiveness to the glass substrate is not improved, and sufficient oxidation resistance may not be acquired to withstand liquid crystal manufacturing process.
Japanese Unexamined Patent Application Publication No. 2005-158887 proposes a copper alloy in which at least one element of titanium (Ti), molybdenum (Mo), nickel (Ni), aluminum (Al) and silver (Ag) is added by 0.5 to weight percent to the copper (Cu). However, the additional element increases electric resistance of the interconnections.
Japanese Unexamined Patent Application Publication No. 2004-91907 discloses the addition of molybdenum (Mo) by 0.1 to 3.0 weight percent to the copper (Cu) and segregation of molybdenum (Mo) to a grain boundary suppresses oxidation by grain boundary diffusion. Although this technique can improve oxidation resistance of the copper (Cu), there is a problem in that the interconnection resistance increases.
International Unexamined Patent Application Publication No. WO2006-025347 discloses that an oxide protective layer formed by an additional element will suppress the oxidation of Cu in the copper alloy layer in which the appropriate additional element is added. The protective layer is formed at an interface of an adjacent insulating layer that suppresses the mutual diffusion. This technique provides a copper interconnection that has high conductivity and good adhesiveness with the substrate. Further, this technique provides liquid crystal display (LCD) devices utilizing this copper interconnections. In addition, this publication suggests that manganese (Mn) is preferable as one of the additional elements. However, this technique is insufficient to realize features of interconnection structures used in the liquid crystal display (LCD) devices and TFT electrode structures.
Japanese patent No. 3302894 proposes a TFT structure used in TFT-LCD devices and explicitly discloses the gate electrode of TFT structure is covered by an oxide layer when a Cu alloy is applied to the gate electrode. This patent discloses that when a first metal is Cu, a second metal is at least one element selected from titanium (Ti), zirconium (Zr), hafnium (Hf), tantalum (Ta), niobium (Nb), silicon (Si), boron (B), lanthanum (La), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), dysprosium (Dy), yttrium (Y), ytterbium (Yb), cerium (Ce), magnesium (Mg), thorium (Th), and chromium (Cr). However, the second element is different from an additional element of the present invention.
None of the above-mentioned documents refers to a structure of source or drain electrodes in a TFT structure. However, high adhesiveness to a semiconductor layer or a pixel electrode, tolerability to a circumstance in which the TFT electrode is used, and stability of electric contacts with source or drain electrodes portion are required for the structure of the source or drain electrode. Therefore, the structure of the source or drain electrode is an important element of liquid crystal display (LCD) device.
As mentioned above, according to these conventional techniques, although adhesiveness to the semiconductor layer or the pixel electrode and the oxidation-resistance layer are tried to be secured by adding an additional alloyed element to the copper (Cu), a sufficient result is not yet obtained in all techniques. Further, sufficient results are not obtained with regard to the high adhesiveness to the semiconductor layer or the pixel electrode and tolerability of circumstances in which the TFT electrode is used. In the same way, the requirement of having stable electric contacts with the source or drain electrodes portion are not yet met.
Especially, although the International Unexamined Patent Application Publication No. WO2006-025347 suggests the liquid crystal display (LCD) device using copper interconnections, a sufficient structure for realizing the gate interconnection structure utilized in the liquid crystal display (LCD) device is not yet achieved by the suggested technique. Further, the Japanese patent No. 3302894 clearly specifies that an oxide layer covering a gate electrode is an oxide layer mainly composed of a second metal element, which is formed by applying a heat treatment in an oxygen atmosphere. However, it is not described at all nor even suggested that the adhesiveness between the semiconductor layer and the source electrode or drain electrode is secured by forming an oxide layer on the source or drain electrodes as a result of reaction between Cu alloy and a Si oxide layer contacting to the Cu alloy by heat treatment, as mentioned in the present invention. Further, an electrically stable contact between the source electrode or drain electrode and the semiconductor layer is not described nor suggested.
In other words, there is a need to provide a solution for all the above-mentioned problems such as, for example, depositing the Cu alloy layer with fewer process steps, decreasing effective resistance of interconnections, and forming a stable electric contact with improving the adhesiveness between the semiconductor layer and the source or drain electrodes. However, these problems cannot be solved by the above-mentioned conventional techniques, therefore actual products, featuring all the requirements, are difficult to be manufactured.
The present invention is made under the above-mentioned situation. The purpose of the present invention is to prevent an oxidation of interconnection materials, including a source electrode or drain electrode, by forming an oxide layer covering the interconnections and securing a high adhesiveness to a semiconductor layer or a pixel electrode. Further, the purpose of the present invention is to provide a liquid crystal display (LCD) device having a TFT structure in which a source electrode or drain electrode is sandwiched between the semiconductor layer, such as amorphous silicon, and a passivation layer with a stable ohmic contact.